Fiber optic sensors, where the fiber is used either as a sensing device or as a transmission medium in connection with an optical sensor, have the advantage that the sensing element is electrically passive so that such devices are suitable for explosive atmospheres, electrically noisy environments or in other situations where heat and/or vibration impede the operation of electrical components.
U.S. Pat. Nos. 4,010,632 and 3,950,987, issued to Slezinger et al., disclose a photoelastic pressure sensor having two physically separate optical channels having a birefringent plate used for phase-shifting a linearly polarized beam by the same amount in opposite directions, illustratively .+-..pi./2.